Abstract The long-term goal of our project is to understand the complex and dynamic interplay between the host and the virus, specifically focusing on HIV-1 integrase (IN) protein-protein interactions. Virally encoded IN enzyme is required for insertion of viral DNA into host chromosomal DNA. Several host factors, including INI1/hSNF5, directly bind to HIV-1 IN. INI1/hSNF5 plays multiple roles during HIV-1 replication. It is selectively encapsidated into HIV-1 virions by its association with IN. Dominant negative mutants of INI1 selectively inhibit HIV-1 particle production. INI1/hSNF5 in the producer cells and that encapsidated in the virions are required for HIV-1 replication. One report using RNA interference analysis indicated that INI1/hSNF5 present in the target cells is inhibitory to HIV-1 replication. INI1 is also required for Tat-mediated transactivation. Our hypothesis is that multiple INI1-mediated effects are due to its varied functions resulting from its ability to associate with many functionally distinct multi-protein complexes. INI1 is a component of the ATP-dependent human chromatin remodeling SWI/SNF complex. Furthermore, our recent studies indicate that INI1 and IN directly bind to SAP18 (Sin3a-associated protein 18kDa), a component of the Sin3a-HDAC(Histone deacetylase)1 complex. Components of Sin3a-HDAC1 but NOT that of SWI/SNF complex are specifically incorporated into HIV-1 virions and are required for early post-entry reverse transcription events in the target cells. Based on these results we hypothesize that virion associated IN-INI1-SAP18-HDAC1 complex is required for reverse transcription in the target cells during HIV-1 replication. The goal of this proposal is to dissect the interplay between INI1-SAP18-IN interactions and determine the influence of HDAC1 complex during HIV-1 replication, especially at reverse transcription. In Aim I, we will segregate proviral and antiviral functions of INI1 by dissecting the role of multiple INI1-mediated associated complexes during HIV-1 replication. By targeting specificity subunits and using catalytic inactive subunits of SWI/SNF and HDAC1 complexes, we will determine how different complexes affect different functions of INI1 during HIV-1. In Aim II, we will carry out genetic and functional analyses to probe INI1-SAP18 and IN-INI1 interactions by: (i) isolating and functionally characterizing SAP18-interaction-defective mutants of INI1;(ii) functionally characterizing panel of IN mutants defective for INI1 interaction and test the recruitment of HDAC1 complex, infectivity and effect on reverse transcription. In Aim III, we will determine the mechanism by which virion-associated HDAC1 complex regulate RT function, by testing the hypothesis that deacetylation of IN or (another substrate) by virion-associated HDAC1 is required for efficient reverse transcription in target cells;and determine the relationship of IN mutants blocked for RT function and recruitment of HDAC1 complex. We hope that our tudies will provide hitherto unanticipated role of HDAC1 complex in HIV-1 replication and shed light on ntricate host-virus dynamic interactions, providing new avenues for anti-HIV-1 strategies.